Method of manufacturing semiconductor device with offset sidewall structure

ABSTRACT

A method of manufacturing a semiconductor device with NMOS and PMOS transistors is provided. The semiconductor device can lessen a short channel effect, can reduce gate-drain current leakage, and can reduce parasitic capacitance due to gate overlaps, thereby inhibiting a reduction in the operating speed of circuits. An N-type impurity such as arsenic is ion implanted to a relatively low concentration in the surface of a silicon substrate ( 1 ) in a low-voltage NMOS region (LNR) thereby to form extension layers ( 61 ). Then, a silicon oxide film (OX 2 ) is formed to cover the whole surface of the silicon substrate ( 1 ). The silicon oxide film (OX 2 ) on the side surfaces of gate electrodes ( 51–54 ) is used as an offset sidewall. Then, boron is ion implanted to a relatively low concentration in the surface of the silicon substrate ( 1 ) in a low-voltage PMOS region (LPR) thereby to form P-type impurity layers ( 621 ) later to be extension layers ( 62 ).

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method of manufacturing asemiconductor device, especially a semiconductor device with an offsetsidewall structure.

2. Description of the Background Art

In conventional semiconductor devices, impurity ion implantation isperformed with gate electrodes as implant masks thereby to formextension layers in a self-aligned manner. The extension layers here areimpurity layers which are formed to produce shallower junctions thanmain source/drain layers later to be formed. The extension layers are ofthe same conductivity type as the main source/drain layers and functionas source/drain layers; thus, they should be referred to as source/drainextension layers but for convenience's sake, they are referred to hereinas the extension layers.

In this method, however, the extension layers extend more than necessaryunder the gate electrodes due to scattering of impurity ions duringimplantation and diffusion of impurity ions in a subsequent process.This is shown in FIG. 34.

In a MOS transistor M1 shown in FIG. 34, a gate insulating film GX isselectively formed on a semiconductor substrate SB and a gate electrodeGT is formed on the gate insulating film GX. In the surface of thesemiconductor substrate SB on both sides of the gate electrode GT, apair of extension layers EX are formed extending under the gateelectrode GT. This state is called a gate overlap. In the case of FIG.34, a gate overlap length of each extension layer EX is represented byL1. As shown, excessive extension of the extension layers EX under thegate electrode GT reduces an effective channel length (L2), therebymaking a short channel effect more prominent.

In recent semiconductor devices with minimum gate lengths of less than0.1 μm, a short channel effect becomes more prominent and a slightreduction of the gate length from the design value will interfere withtransistor operation. That is, the short channel effect has become theleading cause of low manufacturing yield. The gate overlap, which bringsabout a short channel effect, is thus an undesirable phenomenon.

FIG. 35 illustrates in schematic form the MOS transistor M1 in standbymode. As shown in FIG. 35, during standby, a voltage of 0V is applied tothe extension layer EX on the source side, a voltage of 1V to theextension layer EX on the drain side and a voltage of 0V to the gateelectrode GT and the semiconductor substrate SB. In this case, a leakagecurrent flows between the gate and the drain in proportion to the areaof gate-to-drain overlap. In gate insulating films with recentnoticeable tendencies of thin film thickness, gate overlaps produce amore prominent gate-drain current leakage, thereby becoming a factor ofincrease in standby power of LSIs.

FIG. 36 illustrates in schematic form the MOS transistor M1 in operationmode. As shown in FIG. 36, during operation, a voltage of 0V is appliedto the extension layer EX on the source side and a voltage of 0 to 1 Vto the extension layer EX on the drain side and to the gate electrodeGT. The gate and drain voltages may vary in actual circuit operation, inwhich case a large area of gate overlap causes an increase in parasiticcapacitance and requires a greater amount of charge to be appliedthereto, thus becoming a big factor of delay in circuit operation.

To eliminate these problems, offset sidewall structures have recentlybeen adopted. FIG. 37 shows one example of an offset sidewall structure.In FIG. 37, like components to those of the MOS transistor M1 shown inFIG. 34 are designated by the same reference numerals and will not bedescribed herein.

Referring to FIG. 37, an offset sidewall OF is formed adjacent to theside surfaces of the gate electrode GT and the gate insulating film GX.After the formation of the offset sidewall OF, the extension layers EXare formed in a self-aligned manner, using the gate electrode GT and theoffset sidewall OF as implant masks. Thereby the lengths of theextension layers EX extending under the gate electrode GT can bereduced.

In this method, however, the following inconvenience occurs insemiconductor devices with both N-channel MOS transistors (NMOStransistors) and P-channel MOS transistors (PMOS transistors).

FIG. 38 shows an NMOS transistor M11 and a PMOS transistor M12 formed onthe same semiconductor substrate SB.

Referring to FIG. 38, the NMOS transistor M11 comprises a gateinsulating film GX1 selectively formed on the semiconductor substrateSB, a gate electrode GT1 formed on the gate insulating film GX1, anoffset sidewall OF1 formed adjacent to the side surfaces of the gateelectrode GT1 and the gate insulating film GX1, and a pair of extensionlayers EX1 formed in the surface of the semiconductor substrate SB onboth sides of the gate electrode GT1. In this case, the gate overlaplengths of the extension layers EX1 are represented by L3 and aneffective channel length is represented by L4.

The PMOS transistor M12 comprises a gate insulating film GX2 selectivelyformed on the semiconductor substrate SB, a gate electrode GT2 formed onthe gate insulating film GX2, an offset sidewall OF2 formed adjacent tothe side surfaces of the gate electrode GT2 and the gate insulating filmGX2, and a pair of extension layers EX2 formed in the surface of thesemiconductor substrate SB on both sides of the gate electrode GT2. Inthis case, the gate overlap lengths of the extension layers EX2 arerepresented by L5 and an effective channel length is represented by L6.

A comparison between the NMOS transistor M11 and the PMOS transistor M12indicates that the gate overlap length L3 of the NMOS transistor M11 isshorter than the gate overlap length L5 of the PMOS transistor M12 andthus, the effective channel length L4 is longer than L6.

This is because boron (B) which is generally used as source and drainimpurities for PMOS transistors has a much higher diffusion rate withinsilicon than arsenic (As) which is generally used as source and drainimpurities for NMOS transistors.

That is, even if ion implantations of As and B produce implanted layersof the same shape, B will diffuse more widely in a subsequent heattreatment process and thereby the extension layers EX2 of the PMOStransistor M12 have a greater gate overlap length than the extensionlayers EX1 of the NMOS transistor M11.

This results in a more prominent short channel effect of the PMOStransistor M12, an increase in gate-drain parasitic capacitance, and anincrease in gate-drain current leakage.

FIG. 39 illustrates an NMOS transistor (NMOSFET) M21 and a PMOStransistor (PMOSFET) M22 formed on the same semiconductor substrate SB.These transistors M21 and M22 differ from the NMOS transistor M11 andthe PMOS transistor M12 of FIG. 38 in that their respective offsetsidewalls OF11 and OF 12 are greater in width than the offset sidewallsOF1 and OF2, respectively.

By expanding the width of the offset sidewall, the PMOS transistor M22can have a shorter gate overlap length and a longer effective channellength. In the NMOS transistor M21, however, because of the expandedwidth of the offset sidewall OF11, doped impurities cannot extend underthe gate electrode GT1 even by heat treatment during process, no gateoverlaps occur, and thus isolation is established between the source anddrain of the NMOS transistor M21, thereby causing a reduction inoperating current.

Now, as one example of a conventional method of manufacturing asemiconductor device with both NMOS and PMOS transistors, a method ofmanufacturing a semiconductor device with CMOS transistors 90A and 90Bwill be described with reference to FIGS. 40 through 46, which arecross-sectional views illustrating the manufacturing process step bystep. The CMOS transistor 90A is low-voltage compliant and the CMOStransistor 90B is high-voltage compliant, their respective structuresbeing illustrated in the final step of FIG. 46.

Referring first to FIG. 40, an element isolation insulating film 2 isselectively formed in the surface of a silicon substrate 1 to define alow-voltage NMOS region LNR for forming a low-voltage NMOS transistor, alow-voltage PMOS region LPR for forming a low-voltage PMOS transistor, ahigh-voltage NMOS region HNR for forming a high-voltage NMOS transistorand a high-voltage PMOS region HPR for forming a high-voltage PMOStransistor. The low-voltage NMOS and PMOS regions LNR and LPR maygenerically be referred to as a low-voltage circuit portion, and thehigh-voltage NMOS and PMOS regions HNR and HPR may generically bereferred to as a high-voltage circuit portion.

In the surface of the silicon substrate 1, P-well regions PW containingP-type impurities are formed corresponding to the low-voltage NMOSregion LNR and the high-voltage NMOS region HNR, and N-well regions NWcontaining N-type impurities are formed corresponding to the low-voltagePMOS region LPR and the high-voltage PMOS region HPR. In the followingdescription, the P-well regions PW and the N-well regions NW may besimply referred to as the silicon substrate without distinction.

Then, a first insulation film such as silicon oxide film is formed to afirst thickness to cover the whole surface of the silicon substrate 1.After that, a resist mask is formed to expose the low-voltage circuitportion and the first insulation film is removed from the low-voltagecircuit portion by, for example, hydrofluoric acid treatment.

The resist mask is then removed and a second insulation film such assilicon oxide film is formed to a second thickness to cover the wholesurface of the silicon substrate 1. Thereby the low-voltage circuitportion has an insulation film of the second thickness formed thereonand the high-voltage circuit portion has a third insulation film formedthereon which is greater in thickness than the first insulation film.

After a polysilicon layer is formed on the whole surface of the siliconsubstrate 1, the polysilicon layer and the second and third insulationfilms thereunder are patterned to selectively form gate electrodes andgate insulating films in both the low voltage and high-voltage circuitportions. FIG. 40 shows the state after the patterning, wherein in thelow-voltage NMOS region LNR and the low-voltage PMOS region LPR, gateelectrodes 51 and 52 respectively are formed on selectively formed gateinsulating films 3 and in the high-voltage NMOS region HNR and thehigh-voltage PMOS region HPR, gate electrodes 53 and 54 respectively areformed on selectively formed gate insulating films 4.

In the step of FIG. 41, an N-type impurity such as arsenic (As) is ionimplanted to a relatively low concentration into the surface of thesilicon substrate 1 in the high-voltage NMOS region HNR, thereby to forma pair of extension layers 63. FIG. 41 shows that the upper portionother than that of the high-voltage NMOS region HNR is covered with aresist mask RM41 by photolithographic patterning and an N-type impurityis ion implanted into the high-voltage NMOS region HNR with the gateelectrode 53 as an implant mask.

The pair of extension layers 63 are opposed to each other with thesilicon substrate 1 under the gate electrode 53 sandwiched in between.In this case, an area of the silicon substrate 1 under the gateelectrode 53 forms a channel region.

In the step of FIG. 42, a P-type impurity such as boron (B) is ionimplanted to a relatively low concentration into the surface of thesilicon substrate 1 in the high-voltage PMOS region HPR, thereby to forma pair of extension layers 64. FIG. 42 shows that the upper portionother than that of the high-voltage PMOS region HPR is covered with aresist mask RM42 by photolithographic patterning and a P-type impurityis ion implanted into the high-voltage PMOS region HPR with the gateelectrode 54 as an implant mask.

The pair of extension layers 64 are opposed to each other with thesilicon substrate 1 under the gate electrode 54 sandwiched in between.In this case, an area of the silicon substrate 1 under the gateelectrode 54 forms a channel region.

In the step of FIG. 43, a silicon oxide film OX1 is formed to cover thewhole surface of the silicon substrate 1. The silicon oxide film OX1 isthen wholly etched back by anisotropic etching so as to leave thesilicon oxide film OX1 only on the side surfaces of the gate electrodes51 to 54 to form offset sidewalls 9.

In the step of FIG. 44, an N-type impurity such as arsenic (As) is ionimplanted to a relatively low concentration into the surface of thesilicon substrate 1 in the low-voltage NMOS region LNR, thereby to forma pair of extension layers 61. FIG. 44 shows that the upper portionother than that of the low-voltage NMOS region LNR is covered with aresist mask RM43 by photolithographic patterning and an N-type impurityis ion implanted into the low-voltage NMOS region LNR with the gateelectrode 51 and the offset sidewall 9 as implant masks.

The pair of extension layers 61 are opposed to each other with thesilicon substrate 1 under the gate electrode 51 sandwiched in between.In this case, an area of the silicon substrate 1 under the gateelectrode 51 forms a channel region.

In the step of FIG. 45, a P-type impurity such as boron (B) is ionimplanted to a relatively low concentration into the surface of thesilicon substrate 1 in the low-voltage PMOS region LPR, thereby form apair of extension layers 62. FIG. 45 shows that the upper portion otherthan that of the low-voltage PMOS region LPR is covered with a resistmask RM44 by photolithographic patterning and a P-type impurity is ionimplanted into the low-voltage PMOS region LPR with the gate electrode52 and the offset sidewall 9 as implant masks.

The pair of extension layers 62 are opposed to each other with thesilicon substrate 1 under the gate electrode 52 sandwiched in between.In this case, an area of the silicon substrate 1 under the gateelectrode 52 forms a channel region.

In the step of FIG. 46, after an insulation film such as silicon nitridefilm is formed to cover the whole surface of the silicon substrate 1,the silicon nitride film is wholly etched back by anisotropic etching toform sidewall insulating films 11 on the side surfaces of the offsetsidewalls 9.

Thereafter, in the low-voltage NMOS region LNR, using the gate electrode51, the offset sidewall 9 and the sidewall insulating film 11 as implantmasks, an N-type impurity is ion implanted to a relatively highconcentration to form a pair of source/drain layers 81. In thelow-voltage PMOS region LPR, using the gate electrode 52, the offsetsidewall 9 and the sidewall insulating film 11 as implant masks, aP-type impurity is ion implanted to a relatively high concentration toform a pair of source/drain layers 82.

In the high-voltage NMOS region HNR, using the gate electrode 53, theoffset sidewall 9 and the sidewall insulating film 11 as implant masks,an N-type impurity is ion implanted to a relatively high concentrationto form a pair of source/drain layers 83. In the high-voltage PMOSregion HPR, using the gate electrode 54, the offset sidewall 9 and thesidewall insulating film 11 as implant masks, a P-type impurity is ionimplanted to a relatively high concentration to form a pair ofsource/drain layers 84.

Through the aforementioned steps, the semiconductor device with the CMOStransistors 90A and 90B can be obtained.

In conventional techniques, as above described, although the extensionlayers in the low-voltage circuit portion and those in the high-voltagecircuit portion have been formed in different steps, impurity ionimplantations into the PMOS transistor and the NMOS transistor forformation of the extension layers have been performed under the sameimplant conditions.

Thus, the degrees of gate overlaps of the extension layers vary betweenthe NMOS transistor and the PMOS transistor depending on a difference indiffusion rate in the silicon substrate between the N-type impurity (As)and the P-type impurity (B).

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a method ofmanufacturing a semiconductor device with NMOS and PMOS transistors,which is capable of lessening a short channel effect, reducinggate-drain current leakage and reducing parasitic capacitance due togate overlaps, thereby preventing a reduction in the operating speed ofcircuits.

According to the present invention, the method of manufacturing asemiconductor device includes the following steps (a) to (c). The step(a) is to section a major surface of a semiconductor substrate into atleast a first NMOS region for forming a first NMOS transistor and afirst PMOS region for forming a first PMOS transistor. The step (b) isto selectively form a first gate insulating film in both the first NMOSregion and the first PMOS region to form a first gate electrode and asecond gate electrode on the first gate insulating film in the firstNMOS region and the first PMOS region, respectively. The step is to ionimplant an N-type impurity using at least the first gate electrode aspart of an implant mask to form a pair of first extension layers in thesurface of the semiconductor substrate outside a side surface of thefirst gate electrode, and to ion implant a P-type impurity using atleast the second gate electrode as part of an implant mask to form apair of second extension layers in the surface of the semiconductorsubstrate outside a side surface of the second gate electrode. The step(c) includes the step of (c-1) forming first ion-implanted layers by ionimplantation of the N-type impurity and second ion-implanted layers byion implantation of the P-type impurity so that a spacing between thesecond ion-implanted layers is wider than a spacing between the firstion-implanted layers.

In the method of manufacturing a semiconductor device according to thepresent invention, since the spacing between the second ion-implantedlayers formed by ion implantation of a P-type impurity is wider thanthat between the first ion-implanted layers formed by ion implantationof an N-type impurity, the second ion-implanted layers are spaced fromthe second gate electrode. Thus, even if the P-type impurity whichdiffuses more easily is diffused by a subsequent heat treatment process,the gate overlap length of the second extension layers can be preventedfrom being longer than that of the first extension layers. Such aconfiguration can prevent the PMOS transistor from having a moreprominent short channel effect and can prevent a reduction in theoperating speed of circuits due to an increase in gate-drain parasiticcapacitance. It can also prevent an increase in gate-drain currentleakage, thereby inhibiting an increase in standby power consumption.

These and other objects, features, aspects and advantages of the presentinvention will become more apparent from the following detaileddescription of the present invention when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 to 13 are diagrams illustrating the manufacturing process of asemiconductor device according to a first preferred embodiment of thepresent invention;

FIGS. 14 to 28 are diagrams illustrating the manufacturing process of asemiconductor device according to a second preferred embodiment of thepresent invention;

FIGS. 29 to 33 are diagrams illustrating the manufacturing process of asemiconductor device according to a third preferred embodiment of thepresent invention;

FIG. 34 is a diagram showing that extension layers extend more thannecessary under a gate electrode;

FIGS. 35 to 37 are diagrams for explaining problems occurring when theextension layers extend more than necessary under the gate electrode;

FIG. 38 is a diagram of a configuration for preventing excessiveextension of the extension layers under the gate electrode;

FIG. 39 is a diagram for explaining a problem of the configuration forpreventing excessive extension of the extension layers under the gateelectrode; and

FIGS. 40 to 46 are diagrams illustrating the manufacturing process of aconventional semiconductor device.

DESCRIPTION OF THE PREFERRED EMBODIMENTS A. First Preferred Embodiment

<A-1. Manufacturing Method>

As one example of a method of manufacturing a semiconductor deviceaccording to a first preferred embodiment of the present invention, amethod of manufacturing a semiconductor device with CMOS transistors100A and 100B will be described with reference to FIGS. 1 to 13, whichare cross-sectional views illustrating the manufacturing process step bystep. The CMOS transistor 100A is low-voltage compliant and the CMOStransistor 100B is high-voltage compliant, their respective structuresbeing illustrated in the final step of FIG. 13.

Referring first to FIG. 1, an element isolation insulating films 2 isselectively formed in the surface of a silicon substrate 1 thereby todefine a low-voltage NMOS region LNR for forming a low-voltage NMOStransistor, a low-voltage PMOS region LPR for forming a low-voltage PMOStransistor, a high-voltage NMOS region HNR for forming a high-voltageNMOS transistor and a high-voltage PMOS region HPR for forming ahigh-voltage PMOS transistor. The low-voltage NMOS and PMOS regions LNRand LPR may generically be referred to as a low-voltage circuit portion,and the high-voltage NMOS and PMOS regions HNR and HPR may genericallybe referred to as a high-voltage circuit portion.

In the surface of the silicon substrate 1, P-well regions PW containingP-type impurities are formed corresponding to the low-voltage NMOSregion LNR and the high-voltage NMOS region HNR, and N-well regions NWcontaining N-type impurities are formed corresponding to the low-voltagePMOS region LPR and the high-voltage PMOS region HPR. In the followingdescription, the P-well regions PW and the N-well regions NW may bereferred to simply as the silicon substrate without distinction.

Then, a first silicon oxide film with a thickness of 2 to 8 nm is formedto cover the whole surface of the silicon substrate 1. After that, aresist mask is formed to expose the low-voltage circuit portion and thefirst silicon oxide is removed from the low-voltage circuit portion by,for example, hydrofluoric acid treatment.

The resist film is then removed and a second silicon oxide film with athickness of 0.5 to 3 nm is formed to cover the whole surface of thesilicon substrate 1. Thereby the low-voltage circuit portion has thesecond oxide film formed thereon, and the high-voltage circuit portionhas a third insulation film formed thereon which has a thickness of 2 to9 nm greater than that of the first insulation film.

After a polysilicon layer is formed on the whole surface of the siliconsubstrate 1, the polysilicon layer and the second and third insulationfilms thereunder are patterned to selectively form gate electrodes andgate insulating films in both the low-voltage and high-voltage circuitportions. At this time, the minimum gate length is in the range of 0.015to 0.10 μm.

The film thickness of the polysilicon layer is in the range of, forexample, 50 to 200 nm. The polysilicon layer may be replaced by apolysilicon germanium layer or a multilayer structure of a polysiliconlayer and a polysilicon germanium layer. Further, the polysilicon layermay previously be implanted with impurities; or after formation of anundoped polysilicon layer, the undoped polysilicon layer in NMOS regionsmay be implanted with an N-type impurity such as phosphorous (P) and theundoped polysilicon layer in PMOS regions may be implanted with a P-typeimpurity such as boron (B). Of course, the undoped polysilicon may beused without any implantation. The concentration of impurities in thepolysilicon layer should be in the range of 1×10¹⁹ to 1×10²¹ cm⁻³.

FIG. 1 shows the state after patterning, wherein in the low-voltage NMOSregion LNR and the low-voltage PMOS region LPR, gate electrodes 51 and52 respectively are formed on selectively formed gate insulating films 3and in the high-voltage NMOS region HNR and the high-voltage PMOS regionHPR, gate electrodes 53 and 54 respectively are formed on selectivelyformed gate insulating films 4.

In the step of FIG. 2, an N-type impurity such as arsenic (As) is ionimplanted to a relatively low concentration into the surface of thesilicon substrate 1 in the high-voltage NMOS region HNR, thereby to forma pair of N-type impurity layers 631 (extension implantation).

The ion implant conditions for arsenic are an implant energy of 10 to 50keV and a dose of 5×10¹² to 1×10¹⁴ cm⁻². The ion implant conditions forphosphorous (P) are an implant energy of 10 to 30 keV and a dose of5×10¹² to 1×10¹⁴ cm⁻². Alternatively, implantation may be performed witha mixture of those ions.

Then, a P-type impurity such as boron (B) is ion implanted into thesilicon substrate 1 to form a pair of P-type impurity layers 731 (pocketimplantation). The implant conditions at this time are an implant energyof 3 to 15 keV and a dose of 1×10¹² to 1×10¹³ cm⁻².

FIG. 2 shows that the upper portion other than that of the high-voltageNMOS region HNR is covered with a resist mask RM1 by photolithographicpatterning and the extension and pocket implantations are performed onthe high-voltage NMOS region HNR with the gate electrode 53 as animplant mask.

The pair of N-type impurity layers 631 and the pair of P-type impuritylayers 731 grow into a pair of extension layers 63 and a pair of pocketlayers 73 respectively through heat treatment. The pair of extensionlayers 63 are opposed to each other with the silicon substrate 1 underthe gate electrode 53 sandwiched in between. In this case, an area ofthe silicon substrate 1 under the gate electrode 53 forms a channelregion. The pair of extension layers 63 and the pair of pocket layers 73are shown in FIG. 3 and the following figures.

In the pocket implantation, the silicon substrate 1 is inclined at apredetermined angle relative to an axis of implantation and whenimplantation from a predetermined direction is completed, then thesilicon substrate 1 is rotated a predetermined angle for nextimplantation. That is, with intermittent rotation of the siliconsubstrate 1, an N-type impurity may be implanted at an angle into thesilicon substrate 1 outside the side surface of the gate electrode 53.

If we assume that the angle of inclination of the silicon substrate 1 is0° when the silicon substrate 1 is perpendicular to the axis ofimplantation, it should be in the range of 0° to 50°. By inclining thesilicon substrate 1, the pocket layers 73 are formed extending at anangle with respect to the major surface of the silicon substrate 1 andhave their respective tip portions extending under the gate electrode53. It is desirable that the pocket layers 73 extend as far as possibleunder the gate electrode 53; however, even if the inclination angle is0°, i.e., when the axis of implantation is perpendicular to the siliconsubstrate 1, implanted ions will spread out horizontality due toscattering and a subsequent thermal diffusion process and thereby thepocket layers 73 will extend under the gate electrode 53.

Further, ion scattering becomes more prominent as the implantationbecomes deeper and the pocket implantation is performed in a deeperregion than the extension implantation. From this, the pocketimplantation causes a more lateral spread of ions, whereby the extensionlayers 63 are covered with the pocket layers 73.

The pocket layers 73 contain an impurity of the opposite conductivitytype to the source/drain layers and are provided for the purpose ofrestricting the lateral spread of a depletion layer from the drain layerto prevent punch-through. The pocket layers 73, however, increase theimpurity concentration only locally under the gate electrode 53 and thusnever increase the threshold voltage. It should be noted that the pocketimplantation is not an absolute necessity.

In the step of FIG. 3, a P-type impurity such as boron (B) is ionimplanted to a relatively low concentration into the surface of thesilicon substrate 1 in the high-voltage PMOS region HPR, thereby to forma pair of P-type impurity layers 641.

The ion implant conditions for boron are an implant energy of 3 to 20keV and a dose of 5×10¹² to 1×10¹⁴ cm⁻². The ion implant conditions forboron difluoride (BF₂) are an implant energy of 15 to 100 keV and a doseof 5×10¹² to 1×10¹⁴ cm⁻².

Then, an N-type impurity such as arsenic is ion implanted into thesilicon substrate 1 to form a pair of N-type impurity layers 741. Theion implant conditions for arsenic are an implant energy of 40 to 140keV and a dose of 1×10¹² to 1×10¹³ cm⁻². The ion implant conditions forphosphorous are an implant energy of 20 to 70 keV and a dose of 1×10¹²to 1×10¹³ cm⁻². Alternatively, implantation may be performed with amixture of those ions. In the pocket implantation, it is desirable, asabove described, that the silicon substrate 1 be inclined at apredetermined angle relative to the axis of implantation and rotatedintermittently.

FIG. 3 shows that the upper portion other than that of the high-voltagePMOS region HPR is covered with a resist mask RM2 by photolithographicpatterning and the extension and pocket implantations are performed onthe high-voltage PMOS region HPR with the gate electrode 54 as animplant mask.

The pair of P-type impurity layers 641 and the pair of N-type impuritylayers 741 grow into a pair of extension layers 64 and a pair of pocketlayers 74 respectively through heat treatment. The pair of extensionlayers 64 are opposed to each other with the silicon substrate 1 underthe gate electrode 54 sandwiched in between. In this case, an area ofthe silicon substrate 1 under the gate electrode 54 forms a channelregion. The pair of extension layers 64 and the pair of pocket layers 74are shown in FIG. 4 and the following figures.

In the step of FIG. 4, a silicon oxide film OX1 is formed to cover thewhole surface of the silicon substrate 1. The silicon oxide film OX1 hasa thickness of 5 to 30 nm. Then, in the step of FIG. 5, the siliconoxide film OX1 is wholly etched back by anisotropic etching so as toleave the silicon oxide film OX1 only on the side surfaces of the gateelectrodes 51 to 54 to form offset sidewalls 9.

In the formation of the offset sidewalls 9, the etch back of the siliconoxide film OX1 is performed, in which case the silicon substrate 1 canbe etched slightly (a few nanometers). Thus, selective epitaxial growthmay be performed after the formation of the offset sidewalls 9 torestore the silicon substrate 1 removed by etching.

Selective epitaxial growth uses silane gas as a source gas in a CVD(chemical vapor deposition) apparatus at a growth temperature of 500 to800° C., thereby allowing crystal growth of silicon only on siliconlayers such as the source/drain layers. In this case, in order toprevent the growth of silicon on oxide films, a crystal-growth rateshould preferably be maintained at 10 Å/sec or less. It goes withoutsaying that when the etching of the silicon substrate 1 does not presentmuch of a problem, this process is unnecessary.

In the step of FIG. 6, an N-type impurity such as arsenic is ionimplanted to a relatively low concentration into the surface of thesilicon substrate 1 in the low-voltage NMOS region LNR, thereby to forma pair of N-type impurity layers 611.

The ion implant conditions for arsenic are an implant energy of 0.1 to10 keV and a dose of 2×10¹⁴ to 5×10¹⁵ cm⁻².

Then, a P-type impurity such as boron is ion implanted into the siliconsubstrate 1 to form a pair of P-type impurity layers 711. The ionimplant conditions at this time are an implant energy of 3 to 15 keV anda dose of 1×10¹³ to 5×10¹³ cm⁻². In the pocket implantation, it isdesirable, as above described, that the silicon substrate 1 be inclinedat a predetermined angle relative to the axis of implantation androtated intermittently.

FIG. 6 shows that the upper portion other than that of the low-voltageNMOS region LNR is covered with a resist mask RM3 by photolithographicpatterning and the extension and pocket implantations are performed onthe low-voltage NMOS region LNR with the gate electrode 51 and theoffset sidewall 9 as implant masks.

The pair of N-type impurity layers 611 and the pair of P-type impuritylayers 711 grow into a pair of extension layers 61 and a pair of pocketlayers 71 respectively through heat treatment. The pair of extensionlayers 61 are opposed to each other with the silicon substrate 1 underthe gate electrode 51 sandwiched in between. In this case, an area ofthe silicon substrate 1 under the gate electrode 51 forms a channelregion. The pair of extension layers 61 and the pair of pocket layers 71are shown in FIG. 7 and the following figures.

In the step of FIG. 7, a silicon oxide film OX2 is formed to cover thewhole surface of the silicon substrate 1. The silicon oxide film OX2 hasa thickness of 5 to 30 nm. It acts as an offset sidewall on the sidesurfaces of the gate electrodes 51 to 54 and in a later step,unnecessary parts thereof are removed to form offset sidewalls 10.Alternatively, the silicon oxide film OX2 may be etched back at thisstep so that it remains only on the side surfaces of the gate electrodesand the gate insulating film.

In the step of FIG. 8, a P-type impurity such as boron is ion implantedto a relatively low concentration into the surface of the siliconsubstrate 1 in the low-voltage PMOS region LPR, thereby to form a pairof P-type impurity layers 621.

The ion implant conditions for boron are an implant energy of 0.1 to 5keV and a dose of 1×10¹⁴ to 5×10¹⁵ cm⁻². When the extension implantationis performed without removing the silicon oxide film OX2 on the surfaceof the silicon substrate 1, some of implanted boron ions will staywithin the silicon oxide film OX2. Such boron ions in the silicon oxidefilm OX2, however, will be diffused into the silicon substrate 1 by asubsequent heat treatment process and will join the extension layers.

Then, an N-type impurity such as arsenic is ion implanted into thesilicon substrate 1 to form a pair of N-type impurity layers 721. Theion implant conditions at this time are an implant energy of 30 to 120keV and a dose of 1×10¹³ to 5×10¹³ cm⁻². In the pocket implantation, itis desirable, as above described, that the silicon substrate 1 beinclined at a predetermined angle relative to the axis of implantationand rotated intermittently.

FIG. 8 shows that the upper portion other than that of the low-voltagePMOS region LPR is covered with a resist mask RM4 by photolithographicpatterning and the extension and pocket implantations are performed onthe low-voltage PMOS region LPR with the gate electrode 52, the offsetsidewall 9 and the silicon oxide film OX2 on the gate electrode 52 asimplant masks.

The pair of P-type impurity layers 621 and the pair of N-type impuritylayers 721 grow into a pair of extension layers 62 and a pair of pocketlayers 72 respectively through heat treatment. The extension layers 62are opposed to each other with the silicon substrate 1 under the gateelectrode 52 sandwiched in between. In this case, an area of the siliconsubstrate 1 under the gate electrode 52 forms a channel region. The pairof extension layers 62 and the pair of pocket layers 72 are shown inFIG. 9 and the following figures.

In the step of FIG. 9, a silicon nitride film SN1 is formed to cover thewhole surface of the silicon substrate 1. The silicon nitride film SN1has a thickness of 30 to 100 nm.

In the step of FIG. 10, the silicon nitride film SN1 is wholly etchedback by anisotropic etching so as to leave the silicon nitride film SN1on the side surfaces of the gate electrodes 51 to 54, more specifically,on the side surfaces of the offset sidewalls 10 on the side surfaces ofthe gate electrodes 51 to 54 thereby to form sidewall insulating films11.

The offset sidewalls 10 are obtained by, after the etch back of thesilicon nitride film SN1, removing the silicon oxide film OX2 formed onthe gate electrodes 51 to 54 and on the silicon substrate 1.

In the step of FIG. 11, an N-type impurity such as arsenic is ionimplanted to a relatively high concentration into the surface of thesilicon substrate 1 in the low-voltage NMOS region LNR and thehigh-voltage NMOS region HNR, thereby to form a pair of source/drainlayers 81 and a pair of source/drain layers 83, respectively(source/drain implantation).

The ion implant conditions for arsenic are an implant energy of 10 to100 keV and a dose of 1×10¹⁵ to 5×10¹⁶ cm⁻².

After the source/drain implantation, implanted impurity ions areactivated by heat treatment. The heat treat conditions employed hereinare a heat treatment temperature of 800 to 1100° C. and a heat treatmenttime (which is defined as the time during which the maximum temperaturecan be maintained) of 0 to 30 seconds. Even if the heat treatment timeis 0 seconds, the heat treatment can proceed during times until themaximum temperature is reached and until the maximum temperature dropsto room temperatures.

FIG. 11 shows that the upper portion other than those of the low-voltageNMOS region LNR and the high-voltage NMOS region HNR is covered with aresist mask RM5 by photolithographic patterning and the source/drainimplantation is performed on the low-voltage NMOS region LNR using thegate electrode 51, the offset sidewall 9, the offset sidewall 10 and thesidewall insulating film 11 as implant masks and on the high-voltageNMOS region HNR using the gate electrode 53, the offset sidewall 9, theoffset sidewall 10 and the sidewall insulating film 11 as implant masks.

In the step of FIG. 12, a P-type impurity such as boron is ion implantedto a relatively high concentration into the surface of the siliconsubstrate 1 in the low-voltage PMOS region LPR and a high-voltage PMOSregion HPR, thereby to form a pair of source/drain layers 82 and a pairof source/drain layers 84, respectively (source/drain implantation).

The ion implant conditions for boron are an implant energy of 1 to 10keV and a dose of 1×10¹⁵ to 5×10¹⁶ cm⁻². The ion implant conditions forboron difluoride are an implant energy of 5 to 50 keV and a dose of1×10¹⁵ to 5×10¹⁶ cm⁻².

After the source/drain implantation, implanted impurity ions areactivated by heat treatment. The heat treat conditions employed hereinare a heat treatment temperature of 800 to 1100° C. and a heat treatmenttime (which is defined as the time during which the maximum temperaturecan be maintained) of 0 to 30 seconds.

FIG. 12 shows that the upper portion other than those of the low-voltagePMOS region LPR and the high-voltage PMOS region HPR is covered with aresist mask RM6 by photolithographic patterning and the source/drainimplantation is performed on the low-voltage PMOS region LPR using thegate electrode 52, the offset sidewall 9, the offset sidewall 10 and thesidewall insulating film 11 as implant masks and on the high-voltagePMOS region HPR using the gate electrode 54, the offset sidewall 9, theoffset sidewall 10 and the sidewall insulating film 11 as implant masks.

In the step of FIG. 13, a refractory metal film such as cobalt (Co) isformed by sputtering or vapor deposition to cover the whole surface ofthe silicon substrate 1 and then through high-temperature processing at350–600° C., silicide films are formed at junctions between the exposedsurface of the silicon substrate 1 and the refractory metal film andbetween the exposed surfaces of the gate electrodes 51 to 54 and therefractory metal film. Then, the unsilicided refractory metal film isremoved and cobalt silicide films (CoSi₂) 15 and 16 are formed byfurther heat treatment. This completes the formation of the low-voltagecompliant CMOS transistor 100A and the high-voltage compliant CMOStransistor 100B.

<A-2. Function and Effect>

As above described, in the manufacturing method according to the firstpreferred embodiment, the extension layers 61 of the NMOS transistor inthe low-voltage compliant CMOS transistor 100A are formed using the gateelectrode 51 and the offset sidewall 9 as implant masks and theextension layers 62 of the PMOS transistor are formed using the gateelectrode 52 and the offset sidewalls 9 and 10 as implant masks. Thus,the ion-implanted layers 621 formed for the formation of the extensionlayers 62 are more spaced from each other and more away from their gateelectrode than the ion-implanted layers 611 formed for the formation ofthe extension layers 61 are. From this, even if implanted impurity ionsare diffused by a subsequent heat treatment process, the gate overlaplength of the extension layers 62 can be prevented from being longerthan that of the extension layers 61.

Such a configuration can prevent the PMOS transistor from having a moreprominent short channel effect and can also prevent a reduction in theoperating speed of circuits due to an increase in gate-drain parasiticcapacitance. It can also prevent an increase in gate-drain currentleakage, thereby inhibiting an increase in standby power consumption.

Since the extension layers 61 are formed using the gate electrode 51 andthe offset sidewall 9 as implant masks, the ion-implanted layers 611formed for the formation of the extension layers 61 are formed close tothe gate electrode 51. This eliminates the occurrence of a problem thatno overlaps exist because of the extension layers 61 not extending underthe gate and thus isolation is established between the channel and thesource/drain of the NMOS transistor, thereby causing a reduction inoperating current.

According to this preferred embodiment, although the low-voltage CMOStransistor 100A is formed such that the spacing between theion-implanted layers 621 formed for the formation of the extensionlayers 62 becomes greater than that between the ion-implanted layers 611formed for the formation of the extension layers 61, the high-voltagecompliant CMOS transistor 100B is formed by a conventional technique.This is because it is important for CMOS transistors in the high-voltagecircuit portion to maintain hot carrier resistance than reducing a shortchannel effect. That is, a trade-off exists between the reduction of theshort channel effect and the maintenance of the hot carrier resistance;therefore, the high-voltage circuit portion sacrifices the reduction ofthe short channel effect for maintaining the hot carrier resistance.

B. Second Preferred Embodiment

<B-1. Manufacturing Method>

As one example of a method of manufacturing a semiconductor deviceaccording to a second preferred embodiment of the present invention, amethod of manufacturing a semiconductor device with CMOS transistors200A and 200B will be described with reference to FIGS. 14 to 28 whichare cross-sectional views illustrating the manufacturing process step bystep. The CMOS transistor 200A is low-voltage compliant and the CMOStransistor 200B is high-voltage compliant, their respective structuresbeing illustrated in the final step of FIG. 28. In the followingdescription, like components to those described in the first preferredembodiment with reference to FIGS. 1 to 13 are denoted by the samereference numerals or characters and will not be described herein toavoid overlaps.

First, as shown in FIG. 14, through the step of FIG. 1, the gateelectrodes 51 and 52 are formed on the selectively formed gateinsulating films 3 in the low-voltage NMOS region LNR and low-voltagePMOS region LPR, respectively, and the gate electrodes 53 and 54 areformed on the selectively formed gate insulating films 4 in thehigh-voltage NMOS region HNR and the high-voltage PMOS region HPR,respectively.

In the step of FIG. 15, a silicon oxide film OX11 is formed to cover thewhole surface of the silicon substrate 1. The silicon oxide film OX11has a thickness of 5 to 30 nm. In the step of FIG. 16, the silicon oxidefilm OX11 is wholly etched back by anisotropic etching so as to leavethe silicon oxide film OX11 only on the side surfaces of the gateelectrodes 51 to 54 to form the offset sidewalls 9 thereon. After theformation of the offset sidewalls 9, the silicon substrate 1 may berestored by selective epitaxial growth as described in the firstpreferred embodiment.

In the step of FIG. 17, an N-type impurity such as arsenic is ionimplanted to a relatively low concentration into the surface of thesilicon substrate 1 in the high-voltage NMOS region HNR, thereby to formthe pair of N-type impurity layers 631 (extension implantation).

The ion implant conditions for arsenic are an implant energy of 10 to 50keV and a dose of 5×10¹² to 1×10¹⁴ cm⁻². The ion implant conditions forphosphorous are an implant energy of 10 to 30 keV and a dose of 5×10¹²to 1×10¹⁴ cm⁻². Alternatively, implantation may be performed with amixture of those ions.

Then, a P-type impurity such as boron is ion implanted into the siliconsubstrate 1 to form the pair of P-type impurity layers 731 (pocketimplantation). The ion implant conditions at this time are an implantenergy of 3 to 15 keV and a dose of 1×10¹² to 1×10¹³ cm⁻². In the pocketimplantation, it is desirable, as described in the first preferredembodiment, that the silicon substrate 1 be inclined at a predeterminedangle relative to the axis of implantation and rotated intermittently.Further, the pocket implantation is not an absolute necessity.

FIG. 17 shows that the upper portion other than that of the high-voltageNMOS region HNR is covered with a resist mask RM11 by photolithographicpatterning and the extension and pocket implantations are performed onthe high-voltage NMOS region HNR with the gate electrode 53 and theoffset sidewall 9 as implant masks.

The pair of N-type impurity layers 631 and the pair of P-type impuritylayers 731 grow into the pair of extension layers 63 and the pair ofpocket layers 73 respectively through heat treatment. The pair ofextension layers 63 are opposed to each other with the silicon substrate1 under the gate electrode 53 sandwiched in between. In this case, anarea of the silicon substrate 1 under the gate electrode 53 forms achannel region. The pair of extension layers 63 and the pair of pocketlayers 73 are shown in FIG. 18 and the following figures.

In the step of FIG. 18, a P-type impurity such as boron is ion implantedto a relatively low concentration into the surface of the siliconsubstrate 1 in the high-voltage PMOS region HPR, thereby to form thepair of P-type impurity layers 641.

The ion implant conditions for boron are an implant energy of 3 to 20keV and a dose of 5×10¹² to 1×10¹⁴ cm⁻². The ion implant conditions forboron difluoride are an implant energy of 15 to 100 keV and a dose of5×10¹² to 1×10¹⁴ cm⁻².

Then, an N-type impurity such as arsenic is ion implanted into thesilicon substrate 1 to form the pair of N-type impurity layers 741. Theion implant conditions for arsenic are an implant energy of 40 to 140keV and a dose of 1×10¹² to 1×10¹³ cm⁻². The ion implant conditions forphosphorous are an implant energy of 20 to 70 keV and a dose of 1×10¹²to 1×10¹³ cm⁻². Alternatively, implantation may be performed with amixture of those ions. In the pocket implantation, it is desirable, asdescribed in the first preferred embodiment, that the silicon substrate1 be inclined at a predetermined angle relative to the axis ofimplantation and rotated intermittently.

FIG. 18 shows that the upper portion other than that of the high-voltagePMOS region HPR is covered with a resist mask RM12 by photolithographicpatterning and the extension and pocket implantations are performed onthe high-voltage PMOS region HPR with the gate electrode 54 and theoffset sidewall 9 as implant masks.

The pair of P-type impurity layers 641 and the pair of N-type impuritylayers 741 grow into a pair of extension layers 64 and the pair ofpocket layers 74 respectively through heat treatment. The pair ofextension layers 64 are opposed to each other with the silicon substrate1 under the gate electrode 54 sandwiched in between. In this case, anarea of the silicon substrate 1 under the gate electrode 54 forms achannel region. The pair of extension layers 64 and the pair of pocketlayers 74 are shown in FIG. 19 and the following figures.

In the step of FIG. 19, a silicon oxide film OX12 is formed to cover thewhole surface of the silicon substrate 1. The silicon oxide film OX12has a thickness of 5 to 30 nm. The offset sidewalls 9 are integratedwith the silicon oxide film OX12, so that portions of the silicon oxidefilm OX12 on the offset sidewalls 9 are greater in thickness than theother portions.

Then, in the step of FIG. 20, the silicon oxide film OX12 is whollyetched back by anisotropic etching so as to leave the silicon oxide filmOX12 only on the side surfaces of the gate electrodes 51 to 54 to formoffset sidewalls 90 thereon.

In the step of FIG. 21, an N-type impurity such as arsenic is ionimplanted to a relatively low concentration into the surface of thesilicon substrate 1 in the low-voltage NMOS region LNR, thereby to formthe pair of N-type impurity layers 611.

The ion implant conditions for arsenic are an implant energy of 0.1 to10 keV and a dose of 2×10¹⁴ to 5×10¹⁵ cm⁻².

Then, a P-type impurity such as boron is ion implanted into the siliconsubstrate 1 to form the pair of P-type impurity layers 711. The ionimplant conditions at this time are an implant energy of 3 to 15 keV anda dose of 1×10¹³ to 5×10¹³ cm⁻². In the pocket implantation, it isdesirable, as previously described, that the silicon substrate 1 beinclined at a predetermined angle relative to the axis of implantationand rotated intermittently.

FIG. 21 shows that the upper portion other than that of the low-voltageNMOS region LNR is covered with a resist mask RM13 by photolithographicpatterning and the extension and pocket implantations are performed onthe low-voltage NMOS region LNR with the gate electrode 51 and theoffset sidewall 90 as implant masks.

The pair of N-type impurity layers 611 and the pair of P-type impuritylayers 711 grow into the pair of extension layers 61 and the pair ofpocket layers 71 respectively through heat treatment. The pair ofextension layers 61 are opposed to each other with the silicon substrate1 under the gate electrode 51 sandwiched in between. In this case, anarea of the silicon substrate 1 under the gate electrode 51 forms achannel region. The pair of extension layers 61 and the pair of pocketlayers 71 are shown in FIG. 22 and the following figures.

In the step of FIG. 22, a silicon oxide film OX13 is formed to cover thewhole surface of the silicon substrate 1. The silicon oxide film OX13has a thickness of 5 to 30 nm. It acts as an offset sidewall on the sidesurfaces of the gate electrodes 51 to 54 and in a later step,unnecessary parts thereof are removed to form the offset sidewalls 10.Alternatively, the silicon oxide film OX13 may be etched back at thisstep so that it remains only on the side surfaces of the gate electrodesand the gate insulating film.

In the step of FIG. 23, a P-type impurity such as boron is ion implantedto a relatively low concentration into the surface of the siliconsubstrate 1 in the low-voltage PMOS region LPR, thereby to form the pairof P-type impurity layers 621.

The ion implant conditions for boron are an implant energy of 0.1 to 5keV and a dose of 1×10¹⁴ to 5×10¹⁵ cm⁻². When the extension implantationis performed without removing the silicon oxide film OX13 formed on thesurface of the silicon substrate 1, some of implanted boron ions willstay within the silicon oxide film OX13. Such boron ions in the siliconoxide film OX13, however, will be diffused into the silicon substrate 1by a subsequent heat treatment process and then will join the extensionlayers 62.

Then, an N-type impurity such as arsenic is ion implanted into thesilicon substrate 1 to form the pair of N-type impurity layers 721. Theion implant conditions at this time are an implant energy of 30 to 120keV and a dose of 1×10¹³ to 5×10¹³ cm⁻². In the pocket implantation, itis desirable, as previously described, that the silicon substrate 1 beinclined at a predetermined angle relative to the axis of implantationand rotated intermittently.

FIG. 23 shows that the upper portion other than that of the low-voltagePMOS region LPR is covered with a resist mask RM14 by photolithographicpatterning and the extension and pocket implantations are performed onthe low-voltage PMOS region LPR with the gate electrode 52, the offsetsidewall 90 and the silicon oxide film OX13 on the side surface of thegate electrode 52 as implant masks.

The pair of P-type impurity layers 621 and the pair of N-type impuritylayers 721 grow into the pair of extension layers 62 and a pair ofpocket layers 72 respectively through heat treatment. The pair ofextension layers 62 are opposed to each other with the silicon substrate1 under the gate electrode 52 sandwiched in between. In this case, anarea of the silicon substrate 1 under the gate electrode 52 forms achannel region. The pair of extension layers 62 and the pair of pocketlayers 72 are shown in FIG. 24 and the following figures.

In the step of FIG. 24, the silicon nitride film SN1 is formed to coverthe whole surface of the silicon substrate 1. The silicon nitride filmSN1 has a thickness of 30 to 100 nm.

In the step of FIG. 25, the silicon nitride film SN1 is wholly etchedback by anisotropic etching so as to leave the silicon nitride film SN1on the side surfaces of the gate electrodes 51 to 54, more specifically,on the side surfaces of the offset sidewalls 10 on the side surfaces ofthe gate electrodes 51 to 54, thereby to form the sidewall insulatingfilms 11.

The offset sidewalls 10 are obtained by, after the etch back of thesilicon nitride film SN1, removing the silicon oxide film OX13 formed onthe gate electrodes 51 to 54 and on the silicon substrate 1.

In the step of FIG. 26, an N-type impurity such as arsenic is ionimplanted to a relatively high concentration into the surface of thesilicon substrate 1 in the low-voltage NMOS region LNR and thehigh-voltage NMOS region HNR, thereby to form the pair of source/drainlayers 81 and the pair of source/drain layers 83, respectively(source/drain implantation).

The ion implant conditions for arsenic are an implant energy of 10 to100 keV and a dose of 1×10¹⁵ to 5×10¹⁶ cm⁻².

After the source/drain implantation, implanted impurity ions areactivated by heat treatment. The heat treat conditions employed hereinare a heat treatment temperature of 800 to 1100° C. and a heat treatmenttime (which is defined as the time during which the maximum temperaturecan be maintained) of 0 to 30 seconds.

FIG. 26 shows that the upper portion other than those of the low-voltageNMOS region LNR and the high-voltage NMOS region HNR is covered with aresist mask RM15 by photolithographic patterning and the source/drainimplantation is performed on the low-voltage NMOS region LNR using thegate electrode 51, the offset sidewalls 90 and 10, and the sidewallinsulating film 11 as implant masks and on the high-voltage NMOS regionHNR using the gate electrode 53, the offset sidewalls 90 and 10, and thesidewall insulating film 11 as implant masks.

In the step of FIG. 27, a P-type impurity such as boron is ion implantedto a relatively high concentration into the surface of the siliconsubstrate 1 in the low-voltage PMOS region LPR and the high-voltage PMOSregion HPR, thereby to form the pair of source/drain layers 82 and thepair of source/drain layers 84, respectively (source/drainimplantation).

The ion implant conditions for boron are an implant energy of 1 to 10keV and a dose of 1×10¹⁵ to 5×10¹⁶ cm⁻². The ion implant conditions forboron difluoride are an implant energy of 5 to 50 keV and a dose of1×10¹⁵ to 5×10¹⁶ cm⁻².

After the source/drain implantation, implanted impurity ions areactivated by heat treatment. The heat treat conditions employed hereinare a heat treatment temperature of 800 to 1100° C. and a heat treatmenttime (which is defined as the time during which the maximum temperaturecan be maintained) of 0 to 30 seconds.

FIG. 27 shows that the upper portion other than those of the low-voltagePMOS region LPR and the high-voltage PMOS region HPR is covered with aresist mask RM16 by photolithographic patterning and the source/drainimplantation is performed on the low-voltage PMOS region LPR using thegate electrode 52, the offset sidewalls 90 and 10, and the sidewallinsulating film 11 as implant masks and on the high-voltage PMOS regionHPR using the gate electrode 54, the offset sidewalls 90 and 10, and thesidewall insulating film 11 as implant masks.

In the step of FIG. 28, a refractory metal film such as cobalt (Co) isformed by sputtering or vapor deposition to cover the whole surface ofthe silicon substrate 1 and then, through high-temperature processing at350–600° C., silicide films are formed at junctions between the exposedsurface of the silicon substrate 1 and the refractory metal film andbetween the exposed surfaces of the gate electrodes 51 to 54 and therefractory metal film. Then, the unsilicided refractory metal film isremoved and the cobalt silicide films (CoSi₂) 15 and 16 are formed byfurther heat treatment. This completes the formation of the low-voltagecompliant CMOS transistor 200A and the high-voltage compliant CMOStransistor 200B.

<B-2. Function and Effect>

As above described, in the manufacturing method according to the secondpreferred embodiment, the extension layers 61 of the NMOS transistor inthe low-voltage compliant CMOS transistor 200A are formed using the gateelectrode 51 and the offset sidewall 90 as implant masks and theextension layers 62 of the PMOS transistor are formed using the gateelectrode 52 and the offset sidewalls 90 and 10 as implant masks. Thus,the ion-implanted layers 621 formed for the formation of the extensionlayers 62 are more spaced from each other and more away from their gateelectrode than the ion-implanted layers 611 formed for the formation ofthe extension layers 61 are. From this, even if implanted impurity ionsare diffused by a subsequent heat treatment process, the gate overlength of the extension layers 62 can be prevented from being longerthan that of the extension layers 61.

Such a configuration can prevent the PMOS transistor from having a moreprominent short channel effect and can also prevent a reduction in theoperating speed of circuits due to an increase in gate-drain parasiticcapacitance. It can also prevent an increase in gate-drain currentleakage, thereby inhibiting an increase in standby power consumption.

Since the extension layers 61 are formed using the gate electrode 51 andthe offset sidewall 90 as implant masks, the ion-implanted layers 611formed for the formation of the extension layers 61 are formed close tothe gate electrode 51. This eliminates the occurrence of a problem thatno overlaps exist because of the extension layers 61 not extending underthe gate and thus isolation is established between the channel and thesource/drain of the NMOS transistor, thereby causing a reduction inoperating current.

In the low-voltage compliant CMOS transistor 200B, since the extensionlayers 64 of the PMOS transistor are formed using the gate electrode 54and the offset sidewall 90 as implant masks, the ion-implanted layers641 formed for the formation of the extension layers 64 are more spacedfrom each other and more away from the gate electrode. Thus, even ifimplanted impurity ions are diffused by a subsequent heat treatmentprocess, the gate overlap length of the extension layers 64 can beprevented from being longer than required. Accordingly, even a shortchannel effect of the high-voltage compliant CMOS transistor 200B can bereduced, which improves the balance between the maintenance of the hotcarrier resistance and the reduction of the short channel effect.

C. Third Preferred Embodiment

<C-1. Manufacturing Method>

As one example of a method of manufacturing a semiconductor deviceaccording to a third preferred embodiment of the present invention, amethod of manufacturing a semiconductor device with CMOS transistors300A and 300B will be described with reference to FIGS. 29 to 33, whichare cross-sectional views illustrating the manufacturing process step bystep. The CMOS transistor 300A is low-voltage compliant and the CMOStransistor 300B is high-voltage compliant, their respective structuresbeing illustrated in the final step of FIG. 33. In the followingdescription, like components to those described in the first preferredembodiment with reference to FIGS. 1 to 13 are denoted by the samereference numerals or characters and will not be described herein toavoid overlaps.

In the third preferred embodiment, as shown in FIG. 29, through thesteps of FIGS. 1 to 11, the sidewall insulating films 11 are formed onthe side surfaces of the gate electrodes 51 to 54, more specifically, onthe side surfaces of the offset sidewalls 10 on the side surfaces of thegate electrodes 51 to 54. Then, the pair of source/drain layers 81 areformed in the low-voltage NMOS region LNR using the gate electrode 51,the offset sidewalls 9 and 10, and the sidewall insulating film 11 asimplant masks, and the pair of source/drain layers 83 are formed in thehigh-voltage NMOS region HNR using the gate electrode 53, the offsetsidewalls 9 and 10, and the sidewall insulating film 11 as implantmasks.

In the step of FIG. 30, a silicon nitride film SN2 is formed to coverthe whole surface of the silicon substrate 1. The silicon nitride filmSN2 has a thickness of 10 to 50 nm. The silicon nitride film may bereplaced by a silicon oxide film or by a multilayer film of siliconoxide film and silicon nitride film.

In the step of FIG. 31, the silicon nitride film SN2 is wholly etchedback by anisotropic etching thereby to form sidewall insulating films 12on all the side surfaces of the sidewall insulating films 11.

In the step of FIG. 32, a P-type impurity such as boron is ion implantedto a relatively high concentration into the surface of the siliconsubstrate 1 in the low-voltage PMOS region LPR and the high-voltage PMOSregion HPR, thereby to form the pair of source/drain layers 82 and thepair of source/drain layers 84, respectively (source/drainimplantation).

The ion implant conditions for boron are an implant energy of 1 to 10keV and a dose of 1×10¹⁵ to 5×10¹⁶ cm⁻². The ion implant conditions forboron difluoride are an implant energy of 5 to 50 keV and a dose of1×10¹⁵ to 5×10¹⁶ cm⁻².

After the source/drain implantation, implanted impurity ions areactivated by heat treatment. The heat treat conditions employed hereinare a heat treatment temperature of 800 to 1100° C. and a heat treatmenttime (which is defined as the time during which the maximum temperaturecan be maintained) of 0 to 30 seconds.

FIG. 32 shows that the upper portion other than those of the low-voltagePMOS region LPR and the high-voltage PMOS region HPR is covered with aresist mask RM31 by photolithographic patterning and the source/drainimplantation is performed on the low-voltage PMOS region LPR using thegate electrode 52, the offset sidewall 9, the offset sidewall 10 and thesidewall insulating films 11 and 12 as implant masks and on thehigh-voltage PMOS region HPR using the gate electrode 54, the offsetsidewall 9, the offset sidewall 10 and the sidewall insulating films 11and 12 as implant masks.

Then, in the step of FIG. 33, a refractory metal film such as cobalt(Co) is formed by sputtering or vapor deposition to cover the wholesurface of the silicon substrate 1 and then, through high-temperatureprocessing at 350–600° C., silicide films are formed at junctionsbetween the exposed surface of the silicon substrate 1 and therefractory metal film and between the exposed surfaces of the gateelectrodes 51 to 54 and the refractory metal film. Then, the unsilicidedrefractory metal film is removed and the cobalt silicide films (CoSi₂)15 and 16 are formed by further heat treatment. This completes theformation of the low-voltage compliant CMOS transistor 300A and thehigh-voltage compliant CMOS transistor 300B.

<C-2. Function and Effect>

As above described, in the manufacturing method according to the thirdpreferred embodiment, the ion-implanted layers 621 formed for theformation of the extension layers 62 in the low-voltage compliant CMOStransistor 300A are more spaced from each other and more away from theirgate electrode than the ion-implanted layers 611 formed for theformation of the extension layers 61 are. Thus, even if implantedimpurity ions are diffused by a subsequent heat treatment process, thegate overlap length of the extension layers 62 can be prevented frombeing longer than that of the extension layers 61. Further in thelow-voltage compliant CMOS transistor 300A and the high-voltagecompliant CMOS transistor 300B, the ion-implanted layers formed for theformation of the source/drain layers 82 and 84 of the PMOS transistorsare more spaced from each other and more away from their respective gateelectrodes than the ion-implanted layers formed for the formation of thesource/drain layers 81 and 83 of the NMOS transistors. Thus, even ifimplanted impurity ions are diffused by a subsequent heat treatmentprocess, the diffusion of impurity ions from the source/drain layersinto the channel regions can be prevented.

Such a configuration can prevent the PMOS transistor from having a moreprominent short channel effect and can also prevent a reduction in theoperating speed of circuits due to an increase in gate-drain parasiticcapacitance. It can also prevent an increase in gate-drain currentleakage with reliability, thereby inhibiting an increase in standbypower consumption.

Since the extension layers 61 are formed using the gate electrode 51 andthe offset sidewall 9 as implant masks, the ion-implanted layers 611formed for the formation of the extension layers 61 are formed close tothe gate electrode 51. This eliminates the occurrence of a problem thatno overlaps exist because of the extension layers 61 not extending underthe gate and thus isolation is established between the channel and thesource/drain of the NMOS transistor, thereby causing a reduction inoperating current.

While the invention has been shown and described in detail, theforegoing description is in all aspects illustrative and notrestrictive. It is therefore understood that numerous modifications andvariations can be devised without departing from the scope of theinvention.

1. A method of manufacturing a semiconductor device comprising the stepsof: (a) sectioning a major surface of a semiconductor substrate into atleast a first NMOS region for forming a first NMOS transistor and afirst PMOS region for forming a first PMOS transistor; (b) selectivelyforming a first gate insulating film in both said first NMOS region andsaid first PMOS region and forming both a first gate electrode and asecond gate electrode on said first gate insulating film of said firstNMOS region and said first PMOS region, respectively; and (c) ionimplanting an N-type impurity using at least said first gate electrodeof said first NMOS region as part of an implant mask to form a pair offirst extension layers in the surface of said semiconductor substrateoutside a side surface of said first gate electrode of said first NMOSregion, and ion implanting a P-type impurity using at least said secondgate electrode of said first PMOS region as part of an implant mask toform a pair of second extension layers in the surface of saidsemiconductor substrate outside a side surface of said second gateelectrode of said first PMOS region, said step (c) including the stepof: (c-1) forming a pair of first ion-implanted layers by ionimplantation of said N-type impurity in said first NMOS region andforming a pair of second ion-implanted layers by ion implantation ofsaid P-type impurirty in said first PMOS region, wherein a spacingbetween said second ion-implanted layers is wider than a spacing betweensaid first ion-implanted layers, said first and second ion-implantedlayers grow into said first and second extension layers respectivelythrough heat treatment when forming source and drain layers of both saidfirst NMOS transistor and said first PMOS transistor, and said step(c-1) includes the steps of: (c-1-1) forming a first offset sidewallwith a silicon oxide film on side surface of said first and second gateelectrodes; (c-1-2) ion implanting said N-type impurity into said firstNMOS region using said first gate electrode and said first offsetsidewall as implant masks to form said first ion-implanted layers in thesurface of said semiconductor substrate outside the side surface of saidfirst gate electrode; (c-1-3) forming a second offset sidewall on a sidesurface of said first offset sidewall; and (c-1-4) ion implanting saidP-type impurity into said first PMOS region using said second gateelectrode and said first and second offset sidewalls as implant masks toform said second ion-implanted layers in the surface of saidsemiconductor substrate outside the side surface of said second gateelectrode.
 2. The method according to claim 1, wherein said step (a)includes the step of further sectioning said major surface of saidsemiconductor substrate into a second NMOS region for forming a secondNMOS transistor with a higher operating voltage than said first NMOStransistor and a second PMOS region for forming a second PMOS transistorwith a higher operating voltage than said first PMOS transistor; andsaid step (b) includes the step of: selectively forming a second gateinsulating film greater in thickness than said first gate insulatingfilm on both said second NMOS region and said second PMOS region, toform a third gate electrode and a fourth gate electrode on said secondgate insulating film in said second NMOS region and said second PMOSregion, respectively, the method further comprising the steps, performedprior to said step (c), of: forming an offset sidewall on side surfacesof said third and fourth gate electrodes; ion implanting an N-typeimpurity into said second NMOS region using said third gate electrodeand said offset sidewall as implant masks to form a pair of thirdextension layers in the surface of said semiconductor substrate outsidethe side surface of said third gate electrode; and ion implanting aP-type impurity into said second PMOS region using said fourth gateelectrode and said offset sidewall as implant masks to form a pair offourth extension layers in the surface of said semiconductor substrateoutside the side surface of said fourth gate electrode.
 3. The methodaccording to claim 2, wherein said step (c-1-1) includes the step offorming a first insulation film on the whole surface of saidsemiconductor substrate to cover said offset sidewall and forming saidfirst offset sidewall by increasing the thickness of said offsetsidewall.
 4. The method according to claim 1, further comprising thesteps of: (d) forming a first sidewall insulating film on a side surfaceof said second offset sidewall in at least said first NMOS region andsaid first PMOS region; (e) ion implanting an N-type impurity into atleast said first NMOS region using said first gate electrode, said firstand second offset sidewalls and said first sidewall insulating film animplant masks, to form first source/drain layers in the surface of saidsemiconductor substrate outside the side surface of said first gateelectrode; (f) forming a second sidewall insulating film on a sidesurface of said first sidewall insulating film; and (g) ion implanting aP-type impurity into at least said first PMOS region using said secondgate electrode, said first and second offset sidewalls and said firstand second sidewall insulating films as implant masks, to form secondsource/drain layers in the surface of said semiconductor substrateoutside the side surface of said second gate electrode, said steps (d)and (e) being performed after said step (c), said steps (f) and (g)being performed after said step (e).
 5. The method according to claim 1,wherein said step (c-1-1) includes the step of forming a firstinsulating film on the whole surface of said semiconductor substrateand, by selectively removing a first part of said first insulation filmon the surface of said semiconductor substrate by anisotropic etching,leaving a second part of said first insulation film on the side surfacesof said first and second gate electrodes to form said first offsetsidewall, said step (c-1-3) includes the step of forming a secondinsulation film on the whole surface of said semiconductor substrate,said second insulation film formed on the side surface of said firstoffset sidewall forming said second offset sidewall, and said step(c-1-4) includes the step of ion implanting said P-type impurity throughsaid second insulation film and into the surface of said semiconductorsubstrate with said second offset sidewall formed on a side surface ofsaid first offset sidewall and with said second insulation film formedon the surface of said semiconductor substrate.